Thermoplastic polyesters are step growth polymers that are useful when made at high molecular weights. The first step in a common method of producing a polyester or copolyester, such as polyethylene terephthalate, is an esterification or ester-exchange stage where a diacid, typically terephthalic acid, or a diester, typically dimethylterephthalate, reacts with an appropriate diol, typically ethylene glycol, to give a bis(hydroxyalkyl)ester and some oligomers. Water or alcohol is evolved at this stage and is usually removed by fractional distillation.
Pursuant to the goal of making polyethylene terephthalate and other polyesters, a great deal of patent literature is dedicated to describing processes for preparing terephthalic acid/ethylene glycol mixtures suitable as starting materials. In general, these inventions describe specific mixing schemes with a purified terephthalic acid solid and liquid ethylene glycol as starting materials. Additionally, there is a substantial body of literature devoted to producing a purified terephthalic acid in the powder form that is suitable for use in producing PET.
In the present invention, a process to produce polyethylene terephthalate from ethylene oxide and terephthalic acid is provided. Terephthalic acid and ethylene oxide are reacted to form a partially esterified terephthalic acid product. The partially esterified terephthalic acid product for example is subsequently reacted with ethylene glycol in a conventional polyethylene terephthalic process or through the use of a pipe reactor to produce polyethylene terephthalate.
The use of ethylene oxide can be a preferred method over using ethylene glycol. In addition, less heat input can be required during esterification using ethylene oxide versus using ethylene glycol. The polycondensation step can be conducted in one or more stages and can be completed with ethylene glycol addition, if needed, in order to increase the rate and to control the mole ratio. Suitable catalysts and additives can be added prior to or during polycondensation. Possible catalysts include compounds based on Sb, Ge, Ti, Al, Sn and Zr or combinations thereof.